Methods and apparatuses are known for switching of fault currents in synchronism with a power supply system, as is described in CH 443 443 A and EP 938 114 A1, the contents of which are hereby incorporated by reference in their entireties.
In the case of a method as described in CH 443 443 A for disconnection of a fault current which has occurred in an AC power supply system, the disconnection command for opening a high-voltage circuit breaker is delayed until the current, which is oscillating at the power supply system frequency, has the tendency to fall after passing through a current maximum and has fallen below a limit value. It is thus possible to disconnect short circuits with high current amplitudes without them having any effect, and without unacceptably mechanically, electrically and/or thermally loading the circuit breaker.
An apparatus which is already known from EP 938 114 A1 for disconnection in synchronism with the power supply system of a circuit breaker which is arranged in a high-voltage AC power supply system has an appliance which controls the disconnection of the circuit breaker in synchronism with the power supply system, as well as a high-level protective device, which emits a command for disconnection of the circuit breaker when a fault current occurs. The controller is able to identify the fault current and, taking into account the natural response time of the circuit breaker and the next zero crossing of the fault current, to calculate a lead time, after which the disconnection command is passed to the circuit breaker, which is disconnected in synchronism with the power supply system.
U.S. Pat. No. 6 297 569 B1 describes a controllable power supply 10 with a high level of redundancy, with a power switching system 11 and with a control system 12. The power switching system 11 contains two series-connected switches 18 and 20 which are arranged between a current source 15 and a load 22. If one of these two switches can no longer be disconnected, for example because its switching contacts have stuck, then the other switch carries out this disconnection function. For this purpose, voltage sensors 23 and 26 are used to detect voltages which are present on the sides of the switches 18 and 20, respectively, which face the load and which describe the status of the switches 23, 26. The control system 12 is in the form of a microcontroller and has a logic circuit to which the detected voltage signals are supplied, and to which the switching commands 1 are supplied via an input 50. Outputs of the control system 12 act on two driver stages, which respectively have an associated relay K1 for the switch 18 and a relay K2 for the switch 20. In trials runs, in which the switching command 1 as well as the two status signals can be delayed with respect to one another, it is possible to determine in the control device whether both switches are still serviceable or whether the higher-level of the two switches, specifically 18, has stuck. If the control apparatus finds a fault such as this, then it determines that, as an emergency solution, the lower-level switch 20 will then take over the function of the higher-level switch 18.
The known power supply is neither suitable for disconnection of a fault current nor does it have a synchronization controller in which a disconnection command is delayed until the switch can be opened in synchronism with the power supply system, that is to say at a zero crossing of the current to be disconnected. Column 4, lines 36 to 39 just indicates that forms of switching-pulse and waveform peaks occur as well as zero crossings when alternating current is being carried, which the voltage sensors have to cope with and for which solutions exist in the prior art. Column 15, second paragraph relates only to the idea that the higher-level switch 18 carries out the normal current switching processes, and that, and in an emergency when the higher-level switch 18 has failed, the lower-level switch 20 is intended to takeover these switching tasks.